docs: def get_user_agent():
      """
      Construct and informative user-agent string.
      Includes OS version and docker version info
  
      Not thread-safe
      :return:
      """
  
      global user_agent_string
      if user_agent_string is None:
          # get OS type
          try:
              sysinfo = subprocess.check_output('lsb_release -i -r -s'.split(' '))
              dockerinfo = subprocess.check_output('docker --version'.split(' '))
  
              user_agent_string = ' '.join([sysinfo.replace('\t','/'),
                                            dockerinfo.replace(" version ", "/").replace(", ", "/").replace(' ', '/')]).replace('\n','')
          except:
              user_agent_string = None
  
      return user_agent_string